The present invention is directed to novel lead-rich pyrochlore type compounds having the formula EQU Pb.sub.2 [M.sub.2-x Pb.sub.x ]O.sub.7-y (1)
wherein M is selected from Ru, Ir, and mixtures thereof, 0 &lt; x .ltoreq. 1.2 and 0 .ltoreq. y .ltoreq. 1.0. The compounds have many uses including, but not limited to, electrochemical applications. The present invention is also directed to a method of synthesizing these compounds involving solid state reaction at relatively low temperatures in an oxygen-containing environment.
A number of various types of electrochemical devices have been developed over the past few years for the production of electrical energy by electrochemical reaction and obversely for the consumption of electrical energy to effectuate electrochemical reactions. Many of these devices rely upon a reaction involving oxygen (or air) as part of the mechanism to accomplish the desired result. For example, such devices may contain oxygen electrodes which are oxygen reducing cathodes in which oxygen is catalytically electroreduced. Alternatively, such devices may contain oxygen electrodes which catalyze the evolution of oxygen from water. In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, metal-air batteries, fuel cells, electrolyzers, metal electro-winning devices, etc., are among the well-known electrochemical devices which may contain oxygen electrodes. Typically, such devices contain electrocatalysts materials at one or more of their electrodes and precious metals, such as platinum (on carbon support) and silver (on carbon and other supports), are frequently employed as electrocatalysts.
In addition, various electrocatalytic alloys, compounds and compound mixtures have been developed for these electrochemical devices to achieve more desirable systems. For example, U.S. Pat. No. 3,536,533 (Kitamura) describes the use of an alloy of gold, silver, palladium and at least one of platinum, rhodium and ruthenium as a fuel cell electrode electrocatalyst, and U.S. Pat. No. 3,305,402 (Jones et al) describes the use of a combination of platinum and ruthenium oxides as an electrocatalyst. However, both references describe these catalysts as fuel cell anode (or fuel oxidation) catalysts. O'Grady et al. Technical Report No. 37, "Ruthenium Oxide Catalyst for the Oxygen Electrode", Contract No. N0014-67-A-0404-0006 (AD-779-899) Office of Naval Research, May 1974 (National Technical Information Service) describes the use of ruthenium oxide as an electrochemical catalyst for both the generation of oxygen and the reduction of oxygen. U.S. Pat. No. 3,405,010 (Kordesch et al) teaches that spinel type electrode catalysts have been found to produce better activation of the electrode and improved electrolyte repellency of the electrode by the inclusion of ruthenium.
The foregoing prior art describes various types of electrodes including those which utilize iridium and/or ruthenium-containing catalysts. However, none of these references teaches or renders obvious the novel, lead-rich pyrochlore type compounds of the present invention, much less their use as electrocatalysts.
Heretofore, the pyrochlore compounds Pb.sub.2 Ru.sub.2 O.sub.7-y (lattice parameter of 10.253A) and Pb.sub.2 Ir.sub.2 O.sub.7-y (lattice parameter of 10.271A), commonly referred to as lead ruthenate and lead iridate, respectively, have been known. Longo, Raccah and Goodenough, Mat. Res. Bull., Vol. 4, pp. 191-202 (1969) have described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y and their preparation at elevated temperatures which are in excess of 700.degree. C. Sleight, Mat. Res. Bull., Vol. 6, p. 775 (1971) has also described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y (including the pyrochlore compound Pb.sub.2 Ru.sub.2 O.sub.6.5 having a lattice parameter of 10.271A) and their preparation at 700.degree. C and 3000 atmospheres of pressure. U.S. Pat. No. 3,682,840 (Van Loan) describes the preparation of lead ruthenate at temperatures of 800.degree. C and higher. These references do not teach that lead-rich compounds for Formula (1) above exist or that they may be prepared by solid state techniques at temperatures below about 600.degree. C in an oxygen-containing environment. U.S. Pat. Nos. 3,769,382 (Kuo et al) and 3,951,672 (Langley et al) both disclose the preparation of lead ruthenate and lead iridate using various techniques at temperatures of at least about 600.degree. C, and preferably at higher temperatures. However, these references fail to recognize that the lead-rich pyrochlores of the present invention are obtained at generally lower temperatures or that such pyrochlores have improved physical properties. Further, in the event that some lead-rich pyrochlores are inherently and/or incidentally produced at the lower temperatures of the taught methods of these prior art patents, such inherency was not recognized nor were the beneficial properties of such pyrochlores appreciated.
Bouchard and Gillson, Mat. Res. Bull., Vol. 6, pp. 669-680 (1971) describe Bi.sub.2 Ru.sub.2 O.sub.7 and Bi.sub.2 Ir.sub.2 O.sub.7 preparation and properties, including the fact that these compounds have high conductivity and small Seebeck coefficients. However, there is no teaching that the lead-rich compounds of the present invention exist or that they are useful electrocatalysts in electrochemical devices. Derwent's Basic Abstract Journal, Section E. Chemdoc, Week No. Y25, Abstract No. 320 (August 17, 1977), Derwent Accession No. 44866Y/25 describes electrodes for electrolysis of alkaline and carbonate solutions which comprise nickel-plated steel strips coated with high conductivity layers containing Cd.sub.2 Re.sub.2 O.sub.7-y, Pb.sub.2 Re.sub.2 O.sub.7-4 Ni.sub.2 Re.sub.2 O.sub.7. These compounds are prepared by impregnating perrhenic acid and a metal nitrate such as Cd nitrate into a nickel strip and baking at 350.degree. C. However, these compounds are all rhenates rather than ruthenates or iridates and are not taught to be lead-rich. National Bureau of Standards, Wash., D.C. Inst. for Mat. Research, Abstract of Rept. No. NBSIR-75-742 (1975) describes the use of mixed oxides as oxygen-reducing electrocatalysts in acid fuel cells, including the use of barium ruthenate. However, of all materials suggested for such electrocatalysts, none are even pyrochlore type structure compounds, much less the lead-rich type pyrochlore compounds in the present invention. Thus, it should be noted that, for example, barium ruthenate is not a pyrochlore whereas lead ruthenate is a pyrochlore.
In summary, there exists a formidable body of prior art describing the existence of various pyrochlores and their potential uses, and describing various metals and metal oxides as electrocatalyst materials. Notwithstanding such prior art, there is no suggestion or teaching that (a) the lead rich pyrochlore compounds of the present invention even exist, or that (b) the specific pyrochlores of the present invention may be prepared by the claimed method.